This invention relates to a method for making electrical contact materials by the internal-oxidation of silver-solute metals alloys, whereby the solute metals in the silver matrix are converted to metal oxides which afford the alloys a high refractoriness. And, this invention also relates to such electrical contact materials, matrices of which are silver and which are produced in accordance with this invention method.
Silver-tin oxides alloys, matrices of which are silver and solute elements of which are tin which are in turn internal-oxidized to tin oxides, are widely employed today as electrical contact materials.
Similarly, silver-cadmium oxides alloys are known as efficient electrical contact materials, though compared to the silver-tin oxides alloys, they are less employed, because cadmium is harmful to health and because of the prevention of pollution.
However, there is a problem for preparing silver-tin oxides alloys by means of internal oxidation. That is, when a silver matrix of an alloy contains more than about 4.5 weight % of tin, the entire amount of such tin can hardly be internal-oxidized by oxygen which penetrates from the outside of the silver matrix and into the matrix.
In order to solve this problem, it becomes necessary to add to silver-tin alloys such auxiliary solute metals which have faster diffusion velocities and according are capable to carry and convey oxygen towards the inside of silver matrices fast. A typical example of such auxiliary solute metals is indium. In U.S. Patent No. 3,933,485, there is described that silver tin-indium system alloys are internal-oxidized to abtain excellent electrical contact materials. In this U.S. Patent, electrical contact materials which are one of the best electrical contact materials industrially used today, are made by internal-oxidizing silver alloys containing 5-10 weight % of tin and 1.0-6 weight % of indium.
However, even when indium which acts well as the above-mentioned auxiliary solute metal, is added, it is not easy to internal-oxidize tin of more than 4.5% most evenly in a silver matrix. Sometimes, tin oxides tend to segregate excessively about surface areas of the silver matrix, while they are depleted at inner areas. In addition to such drawback, indium oxides are comparatively weak.
Hence, it is desireable to internal-oxidize tin in a silver matrices without employing indium, if possible.